Cemented carbide and cermet powders used for making sintered bodies for e.g. cutting tools for metal machining, wear parts, in mining applications etc. are usually made by first forming a slurry by milling the powder constituents together with binder metal powders, organic binder (e.g. polyethylene glycol) and a milling liquid in either a ball mill or an attritor mill for several hours. The slurry is then usually subjected to a spray drying operation to form granulated cemented carbide or cermet powders which can be used to press green parts that are subsequently sintered.
The main purpose of the milling operation is to obtain a good binder phase distribution and good wettability between the hard constituent grains and the binder phase powder, and in some cases de-agglomerate WC crystals. A good binder phase distribution and good wettability is essential for achieving cemented carbide and cermet materials of high quality. If the phase distribution or wettability is poor, pores and cracks will be formed in the final sintered body which is detrimental for the material. However, obtaining a good binder phase distribution and wettability is very difficult for these types of materials and requires a high input of energy, i.e. quite long milling times, usually 10-40 hours depending on the type of mill used and/or the grade produced. To achieve coarser grain size grades the milling time is relatively low such to minimize WC crystal breakdown whilst trying to ensure good binder distribution.
Ball mills and attritor mills both provide good, homogenous mixing of the powder constituents, binder metal powders and the organic binder. These processes provides a large energy input that can overcome the static friction and binding forces that is required to obtain a good binder phase distribution and good wettability. However, such mills will subject the powders to a milling operation. Hence, the powders, both hard constituent powders and binder metal powders, will partly be grinded so that a fine fraction will be formed. This fine fraction can cause uncontrolled grain growth during the subsequent sintering. Hence, narrow sized raw material can be destroyed by milling.
It is difficult to produce well controlled narrow grain size microstructures since the milling produce a fine fraction that contribute to an uncontrolled grain growth during sintering.
Several attempts have been done to solve this problem. One method designed to obtain a powder comprising a coarse grained WC with a good binder phase distribution, is to deposit a salt, e.g. cobalt acetate, onto the WC-particles, then subjecting the coated WC grains to an elevated temperature thus reducing the cobalt acetate to cobalt. By doing this prior to milling, a good cobalt distribution can be obtained at a reduced grinding time. These types of processes are quite complicated and time consuming. One example of this type of process is described in EP752921B1. Such methods are quite complicated and costly and indeed still require a milling step.
Other types of non-milling mixing methods have also been tested with the aim to avoid the grinding of the powders and thus maintaining properties like grain size of the raw materials.
EP 1 900 421 A1 discloses a process where the slurry is homogenized in a mixer comprising a rotor, a dispersing device and means to circulate the slurry. The dispersion device contains moving parts.
Conventional manufactured WC powder used for cemented carbide is characterized as fairly agglomerated and with different grain shapes and ranges. The non-uniformity of WC powder results from the heterogeneity of the W powder produced by reduction and this can become even more mixed during the subsequent carburization stage. Furthermore, during sintering any WC agglomerates may form larger sintered carbide grains and contain an increased frequency of sigma2 boundaries, i.e. carbide grains together without cobalt layer.
Single crystal WC raw material having an angular or spherical morphology are usually manufactured by being carburized at high temperature and after the W metal has been deagglomerated.
Single crystal WC raw material having an angular or spherical morphology and narrow distribution, are commonly used in applications that requires a superior toughness: hardness relationship e.g. mining applications. In such applications, it is important that the narrow grain size distribution and the morphology are preserved as much as possible.
In order to minimize the milling time, the milling step has been combined with other methods to obtain a good mixing between WC and cobalt.
One object of the present invention is to obtain a homogenous powder blend without milling to form a cemented carbide or cermet body.
Another object of the present invention is to obtain a powder blend where the grain size distribution of the raw materials can be maintained while still obtaining a homogenous powder blend.
Another object of the present invention is to obtain a powder blend using a mixing process that does not contain any moving parts and is subjected to a minimum amount of wear.
It is further an object of the present invention to provide a method making it possible to maintain the grain size, distribution and the morphology of the in the sintered material while still achieving a good mixing.